The following background information may present examples of specific aspects of the prior art (e.g., without limitation, approaches, facts, or common wisdom) that, while expected to be helpful to further educate the reader as to additional aspects of the prior art, is not to be construed as limiting the present invention, or any embodiments thereof, to anything stated or implied therein or inferred thereupon.
Typically, for water chillers, either water cooled or air cooled, of capacity below 200 kW, the refrigerant vapor compressing has been carried out by motor driven positive displacement machines—e.g., pistons, scrolls and screws.
Typically, centrifugal compressors are widely used in large and medium-sized commercial air-conditioning. A centrifugal compressor is formed by an impeller, a diffuser, a volute, a motor, a bearing, a base, and the like. A rotor of the motor is supported by the bearing. The motor drives the impeller. The impeller that rotates at a high speed raises energy of a refrigerant vapor. The diffuser converts kinetic energy into pressure. The volute gathers a compressed gas long a periphery of the diffuser and discharges the gas from the centrifugal compressor. The impeller of the centrifugal compressor is a component that rotates at a high speed in a central air-conditioning system, and reliability of the impeller determines reliability of air conditioning units. Moreover, reliability of the centrifugal compressor is determined by reliability of the bearing supporting the rotor of the centrifugal compressor.
Challenges that miniaturization of centrifugal compressors face are an increase in friction loss resulted from a decrease in Reynolds number, and increases in tip clearance and seal leakage loss. Due to a manufacturing factor, an impeller of a small centrifuge must be an open impeller, and increases in tip clearance and seal leakage loss remarkably exceed an increase in friction loss. Therefore, a necessary approach for improving efficiency of the small centrifuge is to reduce a leakage loss, and an approach for reducing the leakage loss relates to sizes of a tip clearance and a seal clearance, which requires reduction of oscillation of a rotor in a bearing.
The rotation speed of a centrifugal compressor is dictated by the aerodynamic design to maximize the efficiency; a lower tonnage chiller centrifugal compressor has to run at higher rotation speed. A common journal oil bearing is subjected to a sharp increase in loss with an increase in rotation speed. A ball bearing is limited by a rotation speed. Although there is no friction loss, a magnetic bearing is subjected to substantial rotor oscillation in transitional processes such as startup, shutdown and load change, and therefore can hardly be applied to small air conditioning units without sacrificing the efficiency.
Due to an increase in rotation speed and a restriction from strength of a material of a rotor of a motor, a high-speed permanent magnet motor needs to be compact. That is, power density of the motor is significantly improved, and an amount of heat generated per unit volume increases. A traditional manner of adding a water jacket outside a motor housing can hardly cool a stator of the motor effectively, and is of no use in cooling the rotor.
A static bearing is a support structure in the prior art that is of a high rotation speed as well as high axial and radial position accuracy, and has a prospect of being applied to small compressors. Static bearings are sometimes called externally-pressurized bearings, and are classified into hydrostatic bearings and aerostatic bearings; the hydrostatic use liquid and the aerostatic use gas. A static bearing fills a clearance between the bearing and a load (e.g. a rotary shaft) with a high-pressure fluid, so as to form a high-pressure oil film or gas film to support the load. An advantage of the static bearing is that, no matter the load is static or dynamic (rotating), the bearing and the load are separated by the fluid film. Therefore, a device with a static bearing operates smoothly, and involves almost no friction in start, stop, and normal operation of a machine. A disadvantage of the static bearing is that an external pressurization device is required.
The hydrostatic bearing and the aerostatic bearing have unbridgeable differences in designs and structures. If a gas is supplied to a hydrostatic bearing, or a liquid is supplied to an aerostatic bearing, the bearings do not have bearing capacities. Because a hydrostatic bearing uses liquids such as lubricating oil, whereof high density and high viscosity require a relatively larger film thickness. Meanwhile, in the bearing that rotates at a high speed, the high-viscous fluid generates a relatively large loss, causing the bearing to generate heat and to be damaged. Therefore, the hydrostatic bearing is not suitable for a small high-speed centrifugal compressor. On the contrary, a clearance of the aerostatic bearing is very small, and is usually less than 1/10 of that of the hydrostatic bearing, and supplying a gas by using a low-density fluid can reduce a loss and heat generation.
Therefore, it is necessary to provide a small high-efficiency centrifugal compressor, of which a high energy density motor can be effectively cooled, and a rotor has high rotating accuracy as well as high axial position accuracy, so as to ensure safety and high-speed operation of a centrifugal impeller that has a smaller tip clearance and a smaller seal clearance than those in existing compressor technologies.
Other proposals have involved centrifugal compressors that operate with air conditioners. The problem with these systems is that they are not operational with smaller commercial air conditioning units; they can't stabilize rotational and axial movements of a rotor in the motor to reduce a tip clearance and a seal clearance for higher centrifugal compressor efficiency. Even though the above cited centrifugal compressor systems meet some of the needs of the market, a centrifugal compressor that is operational with smaller commercial air conditioning units, and stabilizes rotational and axial movement of a rotor is still desired.